US Western Interior Research Articles

The early Cenomanian crippsi Event at L\xfcneburg (Germany): palaeontological and stratigraphical significance of a widespread Late Cretaceous bioevent

The early Cenomanian crippsi Event comprises a 1–3-m-thick interval characterised by mass occurrences of the early Cenomanian inoceramid Gnesioceramus crippsi, identified in the uppermost Sharpeiceras schlueteri Subzone (lower lower Cenomanian Mantelliceras mantelli Zone), below an interregional sequence boundary (SB Ce 1). At Lüneburg, the event is characterised by densely packed, very large, disc-like valves of G. crippsi. Taphonomy as well as bio- and microfacies suggest an event formation in a deeper shelf setting below the storm-wave base as primary biogenic concentration, the inoceramids living as recumbent forms on a soft substrate in dense populations. When tracked between basins, the stratigraphic pattern of the crippsi Event suggests a moderately prolonged phase (< 100 kyr) of increased shell production with rapid deposition aiding in preserving the shell-rich event strata. Towards the basin margins, it grades into storm wave-reworked bioclastic concentrations. The crippsi Event formed by an interregional population bloom and provides, as an proliferation epibole, an important marker for intra- and interbasinal correlation. The first record of G. mowriensis within the crippsi Event at Lüneburg, hitherto endemic to the US Western Interior Seaway, and the occurrence of the ammonite Metengonoceras teigenense, likewise an endemic North American faunal element, from the level of the crippsi Event in northern France indicate faunal exchange between the New and Old worlds during the early Cenomanian. This faunal dispersal and contemporaneous occurrence of warm-water biofacies in Western Europe during the early Cenomanian is explained by the existence of a perpetual NE-directed current transporting warm surface waters from the Gulf of Mexico towards Europe. The occurrence of short-lived M. teigenense in France allows for the calibration of the uppermost schlueteri Subzone of the mantelli Zone in Europe to the lowermost Neogastroplites muelleri Zone in North America and to assign an age of ~ 98.6–98.7 Ma to the crippsi Event.

Shallow‐water deltaic clinoforms and process regime

AbstractUtilizing two outcrop data sets with dip direction exposures of shallow‐water (tens of meters) deltaic clinoforms, this paper quantifies sedimentary facies proportions and clinoform lengths and gradients, and links process regimes to delta clinoform dimensions. Both data sets are from foreland basins, the Cretaceous Chimney Rock Sandstone of the Rock Springs Formation from the US Western Interior, and the Eocene Brogniartfjellet Clinoform Complex 8 of the Battfjellet Formation from the Central Basin of Spitsbergen. Sedimentary facies indicate presence of both river‐ and wave‐dominated clinothems in each data set. Facies characteristics and distribution implies that river‐dominated clinothem progradation was primarily driven by deposition from weak hyperpycnal flow turbidity currents across the clinoforms, and minor slumps. Wave‐dominated clinothems were constructed by wave processes rather than alongshore currents, and are also progradational subaerial clinoforms, with one exception, where the formation of a compound subaqueous clinoform set indicates erosion and sediment bypass above the wave base. Sediment distribution and lithological heterogeneity in the river‐dominated clinothems is controlled by individual hyperpycnal flow events or mouth‐bar collapse events, and thus by self‐organization and minimal reworking that results in a heterogeneity that is difficult to predict (high entropy). The efficient reworking of river‐derived sediments in wave‐dominated clinothems results in predictable lithological sediment partitioning (low entropy). Clinoform dimension analyses show that although of similar sediment caliber, river‐dominated clinoforms in both data sets are on average 3–4 times steeper and 3–4 times shorter than the wave‐dominated clinoforms, with mean gradients of ca 4 degrees and ca 1 degree, respectively, and mean lengths of 150–230 m and 640–760 m. These results require corroboration from additional data sets, but do suggest that river‐ and wave‐dominated delta clinoforms are likely to have distinct downdip extents (lengths) and gradients for given clinoform heights. Clinoform shape can thus be a method for differentiating ancient river‐ vs. wave‐dominated deltaic clinoforms, in addition to their sedimentary facies, biogenic features and sandstone maturity, and helpful when incorporated into reservoir models.

Ammonoids, their biozonation and their palaeobiogeographic relation across the Turonian-Coniacian boundary in northern Coahuila, Mexico

The Rosario section in northern Coahuila, northeastern Mexico, contains a complete record of distal shelf deposits across the Turonian-Coniacian boundary. Here we describe the ammonoids, the biozonation based on these cephalopods, their taphonomy and some palaeobiogeographic relations. Gaudryceras mite, Peroniceras (P.) tridorsatum, P. (P.) dravidicum, Forresteria (F.) alluaudi, F. (F.) brancoi, F. (F.) peruana, Baculites sweetgrassensis, B. yokoyamai, Neocrioceras maximum, Schlueterella stinnesbecki, Scaphites (S.) frontierensis, S. (S.) preventricosus, S. (S.) sagensis, and S. (S.) uintensis were identified at El Rosario and a nearby complementary outcrop at Carranza. The lower Coniacian Scaphites preventricosus, F. (F.) alluaudi, P. (P.) dravidicum and Neocrioceras maximum zones are represented and provide correlation with sections of the US Western Interior and Europe. The original index for the base of the Coniacian, F. (F.) peruana is also present, although few data exist below the Turonian-Coniacian boundary level. The ammonoids were not transported far before deposition, indicated by their preservation and size distribution. A moderate diversification in open-coiled diplomoceratids seemed to have started in the early Coniacian in this region. The El Rosario section provides a detailed ammonoid record and biozonation, and it is one of very few sections in the world where it can be correlated with the standard inoceramid biozonation.

Stratigraphic complexity in fluvial fans: Lower Eocene Green River Formation, Uinta Basin, USA

AbstractIn this study, measured outcrop sections and geolocated photomosaics are integrated with areal mapping of channel dimensions, degree of amalgamation, calculations of channel‐to‐floodplain ratios and sedimentary facies variability to study and quantify the channel and floodplain deposits in the Sunnyside Delta Interval of the Lower Eocene Green River Formation in the Uinta Basin, Utah. Vertically, sand content and bed thickness increases, due to an increase in the channel‐to‐floodplain ratio, channel size and the degree of channel amalgamation. Laterally, the channel‐to‐floodplain ratio, channel size, the degree of channel amalgamation and the sand content in channel facies decreases in the paleo‐downstream direction. Such vertical and lateral transitions identify the Sunnyside Delta Interval as a fluvial fan (or distributive fluvial system). However, the vertical and lateral transitions occur at multiple spatial scales, demonstrating considerable stratigraphic complexity as compared to the existing facies and architectural models suggested for fluvial megafans and distributive fluvial systems. The smallest‐scale transitions are identified as avulsion‐related packages that form the building blocks of the stratigraphy, whereas the intermediate‐ and largest‐scale transitions are suggested to be related to lobe and whole fan progradation respectively. This documented complexity indicates the significance of self‐organization in building fluvial fan stratigraphy, and demonstrates that changes in the degree of channel amalgamation or in channel‐to‐floodplain ratio are not linked to accommodation changes. On facies scale, an abundance of Froude supercritical‐flow and high‐deposition‐rate facies, in‐channel mud deposits, and in‐channel bioturbation and desiccation indicate deposition in rivers with highly variable discharge. Such discharge conditions suggest seasonally and inter‐annually variable precipitation conditions in the US Western Interior in the Early Eocene.

3-D orientation and distribution of ammonites in a concretion from the Upper Cretaceous Pierre Shale of Montana

One of the most common modes of preservation of ammonites in the Upper Cretaceous US Western Interior is in concretions. We examine an accumulation of ammonites from a single concretion in the lower Maastrichtian Pierre Shale of eastern Montana. The concretion is an oblate spheroid 50 cm in length and 26 cm in diameter, with its long axis parallel to the substrate. It contains approximately 90 ammonite specimens representing three species of Hoploscaphites including adults and juveniles. The concretion also contains other fauna, primarily bivalves and gastropods. A total of 33 ammonites, mostly adults, are concentrated in a cluster that spans 71 % of the length of the concretion (called the “sculpture”). 3-D measurements of the ammonites in the sculpture reveal that (1) the shells dip at all angles, with a significant trend toward more horizontal from west to east; (2) the shells dip with a highly significant bias toward the east, suggesting a current from that direction; and (3) a highly significant number of the shells that are non-vertical face with their left side up. Most of the shells show lethal damage as indicated by missing pieces of body chamber. After settling to the bottom, the shells may have piled up against each other, creating a sediment trap. Other organisms such as scavenging gastropods may have been attracted to the site to feed on the stranded ammonite carcasses. The chambers of the ammonite phragmocones and even some of the body chambers are empty, suggesting relatively rapid burial. Oxygen and carbon isotopic analyses of the ammonite shells reveal that they preserve their original isotopic signature. The values of δ13C of the carbonate cement in the concretionary matrix are much lighter than those in the ammonite shells and imply that cementation of the concretion occurred in association with the decomposition of organic matter.

Durophagous predation on scaphitid ammonoids in the Late Cretaceous Western Interior Seaway of North America

This study is the first to report a trend of predation intensity on scaphitid ammonoids from the Turonian to the Maastrichtian (Late Cretaceous) on the basis of analysis of ventral shell breakage in large samples from the US Western Interior Province. Analysis of 835 adult specimens revealed ventral shell breakage in 50 specimens. In most of the damaged specimens, the breakage occurred in a preferred position at the rear part of the body chamber. Ventral breakage is rare in the Turonian specimens, whereas it is common in the Campanian and Maastrichtian specimens. The shell diameter of adult scaphitid ammonoids tends to increase with time. The position of the breakage and the absence of repairs indicate that the ventral breakage resulted from lethal predation. Based on the incidence of breakage and the size and shape of the breaks, possible predators include fish, reptiles and cephalopods such as Placenticeras, Eutrephoceras and coleoids. Our statistical analysis of ventral shell breakage indicates that the incidence of lethal predation increased in conjunction with an increase in adult shell size, suggesting that the body size of the prey was an important factor in predator–prey interactions. In addition, the predatory damage is more extensive in larger adults.

Inoceramids and biozonation across the Turonian – Coniacian boundary (Upper Cretaceous) at El Rosario, Coahuila, northeastern Mexico

The Rosario section in northern Coahuila, northeastern Mexico, contains a complete record of sediment across the Turonian–Coniacian boundary. Here we describe the inoceramids and the biozonation based on these bivalves. Inoceramus longealatus, Mytiloides herbichi, M. incertus, M. scupini, Didymotis costatus, Cremnoceramus waltersdorfensis waltersdorfensis, C. waltersdorfensis hannovrensis, C. deformis erectus, and C. crassus inconstans, were identified at El Rosario. The uppermost Turonian Mytiloides scupini and C. waltersdorfensis w. zones and the lower Coniacian C. crassus inconstans zones are well represented and are much more expanded than in sections of the US Western Interior and Europe. The index for the base of the Coniacian, C. deformis erectus, is also present, although few data exist to date from the critical Turon ian–Coniacian boundary level. Ammonites are rare, mostly endemic and provide a much lower resolution across the Turonian–Coniacian boundary than inoceramids. We suggest that various acmes observed across the Turonian–Coniacian boundary are driven by local or regional rather than by global causes. The Rosario section provides an expanded sediment and complete inoceramid record across the Turonian–Coniacian boundary.

Testing the congruence of the microfossil versus microfossil record in the Turonian–Coniacian boundary succession of the Wagon Mound–Springer composite section (NE New Mexico, USA)

ABSTRACT Walaszczyk, I., Lees, J.A., Peryt, D., Cobban, W.A. and Wood, C.J. 2012. Testing the congruence of the microfossil versus microfossil record in the Turonian-Coniacian boundary succession of the Wagon Mound-Springer composite section (NE New Mexico, USA). Acta Geologica Polonica, 62 (4), 581-594. Warszawa. The Turonian-Coniacian boundary succession from the Wagon Mound-Springer composite section in the US Western Interior shows a virtually identical macrofaunal record to that revealed in the proposed candidate Coniacian GSSP in the Salzgitter-Salder-Słupia Nadbrzeżna composite section in central Europe, with easy identification in both regions of the base of the Coniacian Stage, as defined by the first appearance of the inoceramid bivalve species, Cremnoceramus deformis erectus (Meek). The macrofaunal boundary definition is additionally confirmed by the foraminiferal and nannofossil data, demonstrating the high potential of the inoceramid marker for the base of the Coniacian. The former claims about distinct diachroneity between macrofossil and microfossil dates in the trans-Atlantic correlations, resulted from methodological deficiencies, and have no factual basis.

Arctic plant diversity in the Early Eocene greenhouse

For the majority of the Early Caenozoic, a remarkable expanse of humid, mesothermal to temperate forests spread across Northern Polar regions that now contain specialized plant and animal communities adapted to life in extreme environments. Little is known on the taxonomic diversity of Arctic floras during greenhouse periods of the Caenozoic. We show for the first time that plant richness in the globally warm Early Eocene (approx. 55-52 Myr) in the Canadian High Arctic (76° N) is comparable with that approximately 3500 km further south at mid-latitudes in the US western interior (44-47° N). Arctic Eocene pollen floras are most comparable in richness with today's forests in the southeastern United States, some 5000 km further south of the Arctic. Nearly half of the Eocene, Arctic plant taxa are endemic and the richness of pollen floras implies significant patchiness to the vegetation type and clear regional richness of angiosperms. The reduced latitudinal diversity gradient in Early Eocene North American plant species demonstrates that extreme photoperiod in the Arctic did not limit taxonomic diversity of plants.

Lower Turonian Record of BelemnitePraeactinocamaxfrom NW Siberia and Its Palaeogeographic Significance

Specimens of the belemnitellid Praeactinocamax Naidin, 1964 are described from the Upper Cretaceous of NW Siberia (Taimyr Region, Lower Agapa River, Russia). The rostra determined as Praeactinocamax aff. plenus consist of an aragonitic anterior part and a calcitic posterior part with a sharp boundary in between. This boundary surface is referred to as the “alveolar fracture”, and it is a typical morphological feature of early belemnitellids and not a result of diagenetic processes. The occurrence of Praeactinocamax in Arctic areas shows a wider palaeobiogeographical distribution of the genus in the Late Cenomanian—Early Turonian interval than previously known. This finding suggests that migration of the late Cenomanian—early Turonian fauna occurred across Turgai channel. The geographic position of these new records may also explain the occurrence of Praeactinocamax in the Turonian of the US Western Interior Seaway, the origin of which has been hitherto unclear.

Modelling of dipping clinoform barriers within deltaic outcrop analogues from the Cretaceous Western Interior Basin, USA

Abstract Deltaic reservoirs typically contain seaward‐dipping surfaces termed clinoforms. Shale and carbonate cements covering clinoforms can frequently form a barrier or baffle to horizontal flow within reservoirs, However, clinoforms are not typically included in static or flow simulation models because they are often not identified in well data and little is known about their 3D geometry. High quality outcrops such as Cretaceous deposits of the US Western Interior Seaway provide an ideal opportunity to study clinoform geometry and shape, and to model their effects on flow. Within this study, two deltaic systems have been studied. The first is the Ferron Delta which crops out in the Wasatch Plateau, central Utah and is a highstand complex comprised of a number of small, overlapping lobes. Clinoforms are common and their 3D geometry is controlled by the position of the lobes. Large growth fault structures within the lobes add to the potential reservoir complexity. The forced regressive Panther Tongue Delta crops out in the Book Cliffs of Utah and is comprised of downstepping lobes with internal clinoforms. Data for modelling included traditional sedimentary logs, photomontages and calibrated photo logs. Models were built in IRAP RMS using a variety of modelling techniques from simple Truncated Gaussian Simulations on a regular grid to object modelling of shale barriers within a dipping grid designed to follow the clinoforms. The models were flow simulated as a means of comparing the different techniques for representing the heterogeneity results show that not modelling clinoforms explicitly in a dipping grid can lead to significant overestimates in the forecasted production; water injection in a down depositional dip position is optimum, and that there are only limited production differences between highstand and lowstand deltas.

High resolution stratigraphic record constrained by volcanic ash beds at the Cenomanian–Turonian boundary in the Western Interior Basin, USA

Five altered volcanic-ash beds have been correlated in the Cenomanian–Turonian boundary interval through the US Western Interior Basin over tens of thousands of square kilometres. Using these independent chronostratigraphic marker-beds to constrain the other classic stratigraphic records such as limestone bed by bed correlations, peaks within the δ 13C isotope excursion and a few other key biological events, a regional correlation scheme has been obtained between seven sections from Arizona to Kansas and northwards to South Dakota. On the basis of this high resolution scheme, we show that the last occurrences of the planktonic foraminiferid Rotalipora cushmani as well as the first occurrence of Helvetoglobotruncana helvetica are strongly diachronous, the Whiteinella archaeocretacea Partial Range Zone, in which the Cenomanian–Turonian boundary is located, being reduced to almost nothing, particularly at Pueblo (Colorado). At this type section, the δ 13C isotope excursion presents strongly negative values in the limestone beds, suggesting that they are of diagenetic origin. In spite of this, the onset and most of the fine-scale features of the carbon-isotope excursion identified at Pueblo are recognised over long distances even in the more coastal sections where bed by bed correlations become more difficult. As suggested by our scheme, differences observed between supposedly continuous sections could be related to hiatuses in the sedimentary record. Such gaps could explain the disappearance of some fine-scale features of the δ 13C isotope excursion. If this interpretation is correct, one hiatus may have been as long as 850 Kyrs.

Inoceramid fauna and biostratigraphy of the upper Middle Coniacian–lower Middle Santonian of the Pueblo Section (SE Colorado, US Western Interior)

The Middle Coniacian to Middle Santonian inoceramid succession of the Pueblo section, southeastern Colorado, US Western Interior, is reviewed in the context of the zonal scheme applied in Europe. The Middle Coniacian record starts with the upper Middle Coniacian Volviceramus involutus Zone, because the base of the succession is characterised by a hitherto unsuspected stratigraphical gap, spanning the topmost Lower and lower Middle Coniacian in European inoceramid terms. The base of the Upper Coniacian is recognised by the entry of Magadiceramus; this substage is divided into a Magadiceramus subquadratus Interval Zone and a Magadiceramus crenelatus Taxon Range Zone. The base of the Santonian is taken at the base of the Cladoceramus undulatoplicatus Taxon Range Zone. The Middle Santonian is assigned to the Cordiceramus bueltenensis Zone, and the base of the Upper Santonian is placed at the base of the zone of Cordiceramus muelleri. The inoceramid faunas of the Pueblo succession are similar to those known from southern Europe, south of the regular occurrence of sphenoceramids. The apparent differences reported in the previous literature between the inoceramid faunas of Europe and of the Western Interior at this level resulted mainly from differences in the taxonomic concepts applied. The new data enable correlation between the proposed inoceramid zonal scheme and the scaphitid ammonite zones.

High-resolution biotratigraphy and chemostratigraphy of the Cenomanian/Turonian boundary event in the Vocontian Basin, southeast France

The Cenomanian/Turonian boundary black shale has been traced in southeastern French subalpine ranges in several sections correlated over tens of kilometres, both on a bed-by-bed basis and with the control of δ 13C isotope curves. Correlations show a thickness increase slopeward and a slight increase in the time span of black shale deposition basinward. They also show that the Vergons section cannot be used as a local reference section for Ocean Anoxic Event 2 (OAE2) because it has probably been affected by synsedimentary sliding. Finally they show that the vertical range of the Whiteinella archaeocretacea Zone changes considerably over short distances. This could be easily understood in widely separated basins but is harder to explain at such a local scale. In addition, using results obtained in the US Western Interior Basin, we question the validity of this Partial Range Zone.

High-resolution stratigraphy of the Cenomanian–Turonian boundary interval at Pueblo (USA) and wadi Bahloul (Tunisia): stable isotope and bio-events correlation

A high-resolution stratigraphy has been developed for the interval encompassing the Cenomanian–Turonian boundary (CTBI), by means of several lithological, biological and geochemical events. This work entails the study of two sections selected on the base of the completeness of their sedimentary record and their contrasting paleogeographical setting: (1) The Rock Canyon Anticline section west of Pueblo, Colorado, (US Western Interior Basin), which is the Global Boundary Stratotype Section and Point (GSSP) candidate for the base of the Turonian stage, as well as the reference section for the ammonite biostratigraphy of the CTBI; and (2) The wadi Bahloul section in Central Tunisia which is the best and complete section spanning this time interval in the southern Tethyan Margin. These sections record similar biogeochemical events that can be correlated over a great distance. Several important biological and geochemical events determined in these sections relative to the ammonite zonation of the CTBI are listed below in chronological order from old to young: 1. FO (first occurrence) of Sciponoceras gracile- Metoicoceras geslinianum ammonite Assemblage-Zone, 2. δ 13C peak I, 3. LO (last occurrence) of Rotalipora cushmani, 4. “ Heterohelix shift”, 5. FO of the ammonites Pseudaspidoceras pseudonodosoides and Neocardioceras juddii, 6. δ 13C peak II, 7. δ 13C peak III, 8. LO of Cenomanian ammonites ( Ps. pseudonodosoides and N. juddii), 9. LO of Anaticinella, 10. FO of Turonian ammonites ( Watinoceras devonense, base of the Turonian stage), 11. “filament event”, 12. FO of Pseudaspidoceras flexuosum, 13. FO of the Tethyan Helvetoglobotruncana helvetica, 14. FO of Mammites nodosoides, 15. FO of the Western Interior H. helvetica. In the two sections, the Cenomanian–Turonian (C/T) boundary, as defined by the ammonite biostratigraphy, is placed within an interval about 50 cm thick. This interval is termed here as the C/T boundary “precision interval”. The δ 13C peak III slightly precedes the precision interval. The genus Anaticinella planktic foraminifer disappears in the middle part of this interval. The “filament event” occurs just above it and is coeval with the first occurrence of Turonian ammonites. These events are useful for placing the C/T boundary precision interval in absence of ammonite markers. Comparing ammonite and planktic foraminiferal biostratigraphy we have dated and correlated changes occurring in planktic foraminiferal assemblages. On this base, as an important result, we have demonstrated the diachroneity of the FO of Helvetoglobotruncana helvetica and the variable duration of the Whiteinella archaeocretacea PRZ.

Paedomorphosis in the Turonian (Late Cretaceous) collignoniceratine ammonite lineage from the north Pacific region

Ontogenetic development and variation of shell ornament of the four Turonian collignoniceratine ammonites, Collignoniceras woollgari (Mantell 1822), Subprionocyclus neptuni (Geinitz 1850), S. normalis (Anderson 1958) and S. minimus (Hayasaka & Fukada 1951) have been compared on the basis of large population samples from Hokkaido (Japan), California and the US Western Interior Province. Our study revealed that the ornament types observed in the early-middle stage of C. woollgari occur in the middle-late stage of the three other species, and that those in the early-middle stage of S. neptuni appear in the middle-late stage of S. normalis and S. minimus. These species are generally regarded as closely related phylogenetically, because they share several common morphological features such as the presence of double ventrolateral tubercles and a serrated keel. If a postulated phylogeny starting from C. woollgari to S. normalis and S. minimus via S. neptuni is accepted, the mode of morphological evolution in the lineages can be explained by paedomorphosis. Available palaeobiogeographical data suggest that this paedomorphic evolution occurred independently on the northwestern and northeastern sides of the northern Pacific, where the C. woollgari – S. neptuni – S. minimus, and C. woollgari – S. neptuni – S. normalis sublineages evolved.

The Upper Turonian of the Bohemian Cretaceous Basin (Czech Republic) exemplified by the Úpohlavy working quarry: integrated stratigraphy and palaeoceanography of a gateway to the Tethys

The north Bohemian Cretaceous (Bohemian Cretaceous Basin; Czech Republic) is located between the northern temperate areas and the Tethys. As a gateway to the Tethys, lithology, fauna and integrated stratigraphy (litho-, bio-, event, sequence, stable isotope stratigraphy) of the Upper Turonian succession in the Úpohlavy working quarry are described. Due to a large hiatus, the δ 13C curve cannot be used for stratigraphic purposes at the base of the section. Up-section, the curve exhibits the well-known middle Upper Turonian positive excursion. Biostratigraphic and sequence stratigraphic subdivisions permit good correlation with other European Cretaceous basins. The inoceramid bivalve assemblage enables correlation with the US Western Interior. Palaeoceanographically, a positive peak in the δ 18O curve above the hiatus indicates the upper part of the Late Turonian cooling event observed in other areas. The cooling was accompanied by a southward shift of a cool/temperate oceanic biosedimentary system, expressed by the sudden turnover from siliciclastic towards pelagic carbonate deposition. It is associated with the establishment of presumed oligotrophic conditions in the immediate vicinity of the West Sudetic Island, as indicated by the sudden decrease of macrofaunal abundance and diversity.

Growth interruptions in silicified conifer woods from the Upper Cretaceous Two Medicine Formation, Montana, USA: implications for palaeoclimate and dinosaur palaeoecology

Taphonomic studies suggest that most dinosaur bone assemblages in the Upper Cretaceous (Campanian) Two Medicine Formation of Montana originated from drought-induced mass mortalities on the floodplain. This hypothesis is tested further by studying intra-annual growth patterns in silicified woods of cupressaceous/taxodiaceous conifers found in association with dinosaur bone material at a palaeolatitude of 48°N. True growth rings are completely absent, but growth interruptions characterised by concentric, variably persistent zones, 1–8 latewood cells wide are common. Interruptions are irregularly spaced so that, for example, four interruptions may occur along a radius of only 25 cells in one specimen, whilst in another specimen a radius more than 650 cells wide may completely lack interruptions. Irregular growth patterns like this occur in Upper Cretaceous woods across a broad latitudinal belt in the US Western Interior demonstrating that they resulted from regional climatic forcing rather than localised phenomena. Growth interruptions record the occurrence of short-term, aperiodic disturbances to tree growth, under an equable, megathermal climate that was otherwise favourable for continuous, year-round cambial activity. The only probable cause of growth interruption formation were drought-induced fluctuations in the water table and resultant water stress. Growth patterns suggest that sufficient water for uninhibited growth was continuously available for years at a time, but when water stress did occur, it did so repeatedly over a very few months. Comparison with present-day East Africa, where trees show very similar growth patterns, implies that the equable, but erratically humid, environment of the Late Cretaceous Western Interior was exactly the kind of setting where mass mortalities amongst megafauna might be expected. Like East Africa, this environment would have had sufficient carrying capacity to permit large herds of megafauna to develop under normal conditions, but when repeated phases of droughts struck, such populations would have been highly vulnerable.

The inoceramid succession across the Campanian – Maastrichtian boundary

Recent study revealed a high correlation potential of inoceramid bivalves across the Campanian – Maastrichtian (Upper Cretaceous) boundary within the Euroamerican biogeographical region. In inoceramid terms the Campanian/Maastricthian boundary, as currently defined, lies in the topmost part of the “Inoceramus” redbirdensis Zone, close to the base of the Endocostea typica Zone. Both zones, as well as the whole late Campanian and early Maastrichtian inoceramid succession is well recognisable across the European sections and across the Atlantic to the US Western Interior succession.

Water mass characteristics in the Cenomanian US Western Interior seaway as indicated by stable isotopes of calcareous organisms

Stable oxygen isotopes taken from taxon-specific planktic foraminifera are potential sources of water mass density proxy data and paleoclimate information. Material suitable for stable isotope analysis is uncommon in the middle to late Cenomanian strata of the Western Interior of the USA. However, well preserved material from 17 correlative horizons at five localities across SE Montana and NE Wyoming has provided an opportunity to obtain reliable δ 18O data from foraminiferal samples, fine-fraction carbonate (e.g. calcareous nannofossils) and inoceramid prisms. With the exception of inoceramids, which for a benthic organism have stable isotopic compositions resembling surface-dwelling calcareous organisms, these data indicate that the water column was vertically stratified and water masses were anomalously depleted in 18O. Within samples, planktic and benthic foraminifera display strong vertical gradients in δ 18O and δ 13C of several per mil. The vertical gradient of δ 18O ranges from more negative nearer the sea surface to less negative at the sea floor, with a gradient of about 4‰. This implies less saline/warmer waters near the surface with increasingly more saline/colder waters with depth. This isotopic distribution indicates a stably stratified water column at least seasonally. In addition, a persistent vertical carbon isotope gradient of 2–3‰ suggests a strong nutrient gradient that if mixed would result in high surface water fertility. Preservation of high amounts of organic carbon in the sediments corroborates this inference. Although the water column was stratified overall, the nutrients necessary to promote high productivity were clearly renewed at least seasonally. Nutrient renewal could have been achieved by (1) winter cooling of the upper layer allowing deeper seasonal mixing with nutrient-rich deep waters, (2) seasonal shallowing of the pycnocline into the photic zone, (3) entrainment and decomposition of organic matter at the previously proposed frontal convergence, (4) upwelling of nutrient-rich deep waters along the previously proposed oceanic front, or (5) mixing of ‘shelf’ waters with more oceanic waters across an oceanic front.